Monitoring fluid in a subject using an electrode configuration providing negative sensitivity regions

ABSTRACT

An amount of fluid in a thoracic or other region of a subject may be monitored by internally injecting an electrical energy stimulus (e.g., constant voltage source) through the region, detecting voltage resulting from the electrical energy stimulus, and calculating a fluid volume indicative signal. The injected energy stimulus creates a first lead field. The responsive voltage is detected using an electrode configuration that defines a second lead field, which is arranged in a negative sensitivity configuration with respect to the first lead field at the region being monitored.

TECHNICAL FIELD

This patent document pertains generally to measuring an amount of fluidin a region of a subject, such as the thoracic region. Moreparticularly, but not by way of limitation, this patent documentpertains to monitoring fluid in a region of a subject using an electrodeconfiguration with negative sensitivity and methods related thereto.

BACKGROUND

Variations in how much fluid is present in a subject's thoracic regioncan take various forms and can have different causes. As one example,eating salty foods can result in the retainment of excessive fluid inthe thorax, which is commonly referred to as “thoracic fluid,” andelsewhere. Another source of fluid build-up in the thorax is pulmonaryedema, which involves a build-up of extravascular fluid in or around thelungs.

One cause of pulmonary edema is congestive heart failure (referred to as“CHF”), which is also sometimes referred to as “chronic heart failure,”or simply as “heart failure.” CHF may be conceptualized as an enlargedweakened heart muscle. The impaired heart muscle results in poor cardiacoutput of blood. As a result of such poor blood circulation, blood tendsto pool in blood vessels in the lungs and becomes a barrier to normaloxygen exchange. In brief, pulmonary edema may be an indicative andimportant condition associated with CHF.

Pulmonary edema, if it exists, may present a medical emergency thatrequires immediate care. While it can sometimes prove fatal, the outlookfor subjects possessing pulmonary edema can be good upon early detectionand prompt treatment of the same. If left undetected (and consequentlyuntreated), pulmonary edema may lead to death.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.The drawings illustrate generally, by way of example, but not by way oflimitation, various embodiments discussed in the present document.

FIG. 1 is a block diagram illustrating exemplary causes and indicationsof abnormal fluid build-up in a subject's lungs, such as may be theresult of pulmonary edema.

FIG. 2 is a graph illustrating a trend in calculated thoracic impedancethat may indicate an increased fluid build-up in a subject's lungs orother organ.

FIG. 3 is a perspective view illustrating at least a portion of anexemplary system adapted to monitor thoracic fluid in a subject, thesystem including an electrode configuration providing positivesensitivity regions within the subject.

FIG. 4 is a perspective view illustrating at least a portion of anexemplary system adapted to monitor thoracic fluid in a subject, thesystem including an electrode configuration providing negativesensitivity regions within the subject.

FIG. 5 is a graph illustrating a trend in sensed voltage resulting froman injected stimulus that may indicate an increased fluid build-up in asubject's lungs.

FIG. 6 is a block diagram illustrating one or more components of animplantable medical device, which may be used in a system including anelectrode configuration providing negative sensitivity regions within asubject.

FIG. 7 is a perspective view illustrating a portion of an exemplarysystem adapted to monitor thoracic fluid in a subject and opposing leadfields associated with a negative sensitivity electrode configuration.

FIG. 8 is a flow chart illustrating an exemplary method providingmonitoring of fluid in a region of a subject.

DETAILED DESCRIPTION

The following detailed description includes references to theaccompanying drawings, which form a part of the detailed description.The drawings show, by way of illustration, specific embodiments in whichthe present systems and methods may be practiced. These embodiments,which are also referred to herein as “examples,” are described in enoughdetail to enable those skilled in the art to practice the presentsystems and methods. The embodiments may be combined, other embodimentsmay be utilized or structural, electrical, or logical changes may bemade without departing from the scope of the present systems andmethods. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present systems andmethods are defined by the appended claims and their legal equivalents.

In this document, the terms “a” or “an” are used to include one or morethan one; the term “or” is used to refer to a nonexclusive “or” unlessotherwise indicated; the term “subject” is used to include the term“patient”; and the term “thorax” refers generally to a subject's bodybetween the neck and the diaphragm. In addition, it is to be understoodthat the phraseology or terminology employed herein, and not otherwisedefined, is for the purpose of description only and not of limitation.

Furthermore, in the event of inconsistent usages between this documentand those documents so incorporated by reference, the usage in theincorporated references should be considered supplementary to that ofthis document; for irreconcilable inconsistencies, the usage in thisdocument controls.

Introduction

In general, edema (i.e., an excess fluid buildup in a region of asubject) is a failure or decompensation of one or more homeostaticprocesses within a subject's body. The body normally prevents thebuild-up of fluids therewithin by maintaining adequate pressures andconcentrations of salt and proteins, and by actively removing excessfluid. If a disease affects any of these normal bodily mechanisms or ifthe normal bodily mechanisms are unable to keep up with the fluidbuild-up, the result may be edema, such as pulmonary edema.

There are several conditions or diseases that may cause or affectpulmonary edema. As shown in FIG. 1, this includes, among others, heartfailure 102, left-sided myocardial infarction 104, high blood pressure106, altitude sickness 108, emphysema 110, cancers that affect thelymphatic system 112, diseases that disrupt protein concentrations 114,or epithelial pathologies 115, such as those caused by inhalation oftoxic chemicals, leading to flooding of the alveoli. While pulmonaryedema 100 may be a sign of many conditions or diseases, the prospectthat pulmonary edema 100 may be a sign of failing heart circulation isoften of first concern to caregivers (e.g., health care professionals)due to the severity of its nature.

Unfortunately, the first indication that an attending caregivertypically has of an occurrence of pulmonary edema 100 is very late inthe disease process, such as when it becomes a physical manifestationwith swelling 118, noticeable weight gains 120, jugular venousdistension 126, or breathing difficulties 122 so overwhelming as to benoticed by the subject who then proceeds to be examined by his/hercaregiver. For a heart failure subject, hospitalization at such a(physically apparent) time would likely be required.

Today, heart failure is a major cause of hospital admissions. A portionof these admissions is due to fluid accumulation in the lungs as aresult of pulmonary edema 100, which is challenging to treat and oftengoes unrecognized until a subject is critically ill. It is not unusualfor subjects with heart failure to require hospitalization or urgenttreatment at an emergency room or critical care unit. It is estimatedthat approximately 30-40% of subjects with heart failure arehospitalized every year. Further, heart failure is a leadingdiagnosis-related group among hospitalized subjects over the age of 65.

Morbidity and mortality of heart failure can potentially be lowered withtimely detection and appropriate treatment of disease conditions intheir early stages, such as upon early detection and treatment ofpulmonary edema 100. Early detection and treatment of pulmonary edema100 may reduce or eliminate the need for hospital admission of subjectswith heart failure. A reduction or elimination of the need forhospitalization results in lower health care costs. It is currentlyestimated that overall expenditures for management and treatment ofheart failure may be as high as 24 billion dollars or more per year.

In an effort to detect impending edema and avoid its associatedhospitalizations, the present systems and methods utilize concepts oflead field theory. In brief, a lead field can be used to describe acurrent density vector field that results when a unit of current isapplied between at least two electrodes. “Lead field” is a concept thatapplies to the electrodes injecting current (“current lead field”) aswell as to those electrodes measuring resulting voltage (“voltage leadfield”). Although a lead field associated with the voltage measurementelectrodes may seem surprising at first, as voltage measurement does notentail the injection of current and therefore the creation of anassociated lead field in the body, it is sometimes convenient totheoretically conceptualize a current density field resulting fromenergizing the voltage measurement electrodes with a unit of current.

In designing electric systems that monitor fluid via tissue resistivitychanges, it is useful to arrange electrodes in the body so that thecurrent and voltage lead fields intersect at a targeted region withdesired geometries and orientations. This allows for high sensitivity ina particular organ (e.g., the lung) or simplification of the circuitryof a monitoring system, thereby potentially reducing its cost. It ispossible to arrange the electrodes to create regions within a subjectthat have positive sensitivity. In a positive sensitivity region, anincrease in fluid results in a corresponding decrease in the monitoredvoltage and impedance. It is also possible to arrange the electrodes tocreate regions within the subject that have negative sensitivity, inwhich an increase in fluid results in an increase in the monitoredvoltage and impedance.

The negativity or positivity of sensitivity in the monitored region is acharacteristic of the dot product of the current and voltage lead fieldsat the desired region's location. For example, in a four electrodesystem with two electrodes injecting a test current and two otherelectrodes measuring a resulting voltage, if the current and voltagelead fields have opposing directions (e.g., an angle between the leadfield lines is greater than 90 degrees) at the region of interest, suchregion will be a negative sensitivity region (see, e.g., FIG. 4, wherethe left lung 304 is located in a region of negative sensitivity). Onthe other hand, if the fields are parallel or substantially parallel(e.g., an angle between the lead field lines is less than 90 degrees),then the region will have positive sensitivity (see, e.g., FIG. 3, wherethe left lung 304 is located in a positive sensitivity region).

With the above discussed lead theory in mind, the present systems andmethods may advantageously provide enhanced detection of pulmonary edema100 (FIG. 1) or other abnormal fluid build-up, such as by providingincreased sensitivity or by providing more simple monitoring, which maybe less costly to implement. This may provide a more timely or cheaperindication of heart failure. As one example, increased fluid within asubject may be monitored using an increase in a measured voltageresulting from an internally injected electrical energy stimulus 124(FIG. 1) (e.g., a constant voltage source given by, for example, aleading edge part of a capacitor discharge pacing pulse) when electrodesare arranged such that the lung or other region of interest issubstantially located in a negative sensitivity region.

EXAMPLES

Positive Sensitivity

As discussed in commonly assigned Belalcazar, U.S. patent applicationSer. No. 11/419,120, entitled “MONITORING FLUID IN A SUBJECT USING AWEIGHT SCALE,” filed even date herewith, which is hereby incorporated byreference in its entirety, it is possible to monitor fluid in a thoracicregion 302 (FIG. 3) by making one or more electrical impedancemeasurements across a subject's lungs 304 (FIG. 3) (such measurementscommonly referred to as “thoracic impedance”). One exemplary techniquefor measuring thoracic impedance includes, among other things,internally injecting a (typically constant) current through thoracicregion 302 using an implantable medical device (referred to as “IMD”)306 (FIG. 3) having at least one electrode associated therewith,detecting a resulting voltage using one or both of IMD 306 or anexternal device having at least one other electrode associatedtherewith. This measurement may be carried out in a manner such that thelead fields associated with the current injection electrodes andresulting voltage measurement electrodes are substantially parallel atthe organ being targeted for monitoring. An impedance value can becalculated by taking the ratio of resulting voltage to injected current.That is, the thoracic impedance (Z) may be determined from Ohm's law(Z=resulting voltage/injected current).

Because internal organs, such as left lung 304, have electricalresistance, electric field laws predict that a flow of current (e.g.,injected current) will result in a voltage across the organs in asubject. As fluid content in the organ increases, the resistivity of theorgan decreases, and, for a given current, the resulting voltage alsodecreases. As a result of the voltage decreasing, thoracic impedance 116will also decrease. In this way, a reduction in thoracic impedance 116indicates the presence of an increase in fluid within an organ, such asthe lungs 304 (FIG. 3). Conversely, a fluid decrease in the lungs 304corresponds to an increase in thoracic impedance sensed. FIG. 2illustrates a general decrease 200 in thoracic impedance (Z) as timeprogresses, such as from time period t₁₀₀-t₁₀₈, and thereby indicates anincrease in fluid in the thoracic cavity during such period, which maybe the result of pulmonary edema 100. Initially, such as from timeperiod t₀-t₉, FIG. 2 illustrates a substantially stable fluid balancecondition as the thoracic impedance trends horizontally 202.

FIG. 3 shows an example of an electrode configuration with positivesensitivity with respect to a subject's left lung 304, that yields suchan antagonistic relationship between resulting voltage and injectedcurrent. In such a system, the magnitude of the current and voltage tendin opposite directions with an increase in left lung 304 fluid. In FIG.3, associated lead fields substantially align with one another at theleft lung (i.e., intersect at angles less than 90 degrees). In thisexample, an implantable apparatus including an IMD 306 and at least onelead 314 electrically coupled thereto are shown within a cut-away areaof a subject. The at least one lead 314 extends from a lead proximal end316, where it is coupled to IMD 306, to a lead distal end 318 disposedwithin, over, or about a heart 322 and thereby provides at least oneconductive path from IMD 306 to heart 322. As shown, but as may vary,the at least one lead 314 includes two implanted electrodes 308, 310disposed near lead distal end 318, while a housing 312 or header portionof the IMD 306 acts as a third implanted electrode by being at leastpartially conductive (typically referred to as a “can” electrode).

In this way, when IMD 306 provides an electrical energy stimulus(provided by, for example, a dedicated constant current source circuit),lead 314 and one of electrodes 308 or 310 deliver the stimulus throughone or more internal organs as an injected current, which may return tothe IMD 306 by way of conductive housing 312. As shown, a lead field 320is associated with the current-injecting electrodes. The injectedcurrent results in a voltage being created, such voltage beingmeasurable using the other one of electrodes 308 or 310 together withthe housing electrode 312. As shown, a lead field 324 is associated withthe resulting voltage measurement electrodes.

In traditional impedance systems which use positive sensitivityarrangements, what is sought with electrode positioning is to maximizeat the target organ the so-called “dot product” of the current andvoltage lead fields. In the example of FIG. 3, the electrodes associatedwith the electrical energy stimulus (i.e., the injected current) definea first lead field in the thoracic region, while the electrodesassociated with the resulting voltage define a second lead field. Inplacing the electrodes optimally, one may seek to increase the dotproduct in the organ of interest, such as by decreasing the angle ofintersection between vectors of the current and voltage lead fields.Increasing the magnitudes of the current and voltage lead fields in theorgan or region of interest will also tend to increase the dot productat that organ. The current and voltage lead fields depend on theelectrode positioning as well as on the internal distribution andproperties of tissues. As shown in FIG. 3, the lead field associatedwith the electrical energy stimulus electrodes 310, 312 and the leadfield associated with the resulting voltage measurement electrodes 308,312 are in a similar orientation and approximately parallel, andtherefore, tend to maximize the dot product in the region around heart322 and left lung 304. Due to the parallel same-orientation nature ofthe associated fields at the left lung, such an electrode configurationmay be referred to as an electrode arrangement with positive sensitivitywith respect to the left lung 304.

Negative Sensitivity

The dot product at a given thoracic region can also be negative, andtherefore, so can the sensitivity of the monitoring system to thatregion. A negative dot product, and thus a negative sensitivity, occursin regions (e.g., 406 (FIG. 4)) where the field vectors (e.g., 402, 404(FIG. 4)) associated with the electrical energy stimulus electrodes andthe resulting voltage measurement electrodes have components in oppositedirections (e.g., where the lead fields intersect at angles greater than90 degrees). In regions with negative sensitivity, an increase in fluiddecreases tissue resistivity, but will actually cause an increasedvoltage and impedance. That is, in a negative sensitivity arrangementwith a constant voltage source, for example, injected current andmeasured voltage change synergistically with an increase in fluid (asopposed to antagonistically, such as in a positive sensitivityarrangement). Therefore, it is actually detrimental to use impedance inthis case, as taking the ratio of voltage to current will actuallydiminish the resulting physiological signal. Computer modeling indicatesthat, in a negative sensitivity arrangement, delivering a constant testvoltage and monitoring a resulting voltage is sufficient for fluidmonitoring.

FIG. 4 illustrates one exemplary negative sensitivity electrodearrangement having associated lead fields that substantially oppose oneanother at the left lung 304. In this example, an implantable apparatusincludes an IMD 306, a first lead 314, and a second lead 408. First lead314 extends from a lead proximal end 316, where it is coupled to IMD306, to a lead distal end 318 disposed within, over, or about a heart322 (such as a right ventricle 410) and thereby provides at least oneconductive path from IMD 306 to the right ventricle. Second lead 408extends from a lead proximal end 414 to a lead distal end 416 disposedwithin, over, or about heart 322 (such as a left ventricle 412) andthereby provides a conductive path from IMD 306 to the left ventricle.As shown, but as may vary, first lead 314 includes two (implanted)electrodes 308, 310 disposed near lead distal end 318, second lead 408includes one (implanted) electrode 420 disposed near lead distal end416, and a housing 312 or header of IMD 306 acts as a fourth (implanted)can electrode by being conductive or at least partial conductive.

In this way, when IMD 306 provides an electrical energy stimulus (e.g.,from a constant voltage source given by, for example, a leading edgepart of an applied pacing pulse via electrical energy output circuit 602(FIG. 6)), lead 314 and electrode 310 deliver the stimulus through oneor more internal organs as an injected current, which may return to theIMD by way of conductive housing 312. As shown, a lead field 402 isassociated with the electrical energy stimulus electrodes. Theelectrical energy stimulus results in a responsive voltage measured(i.e., sensed) using electrodes 308 and 420. As shown, a lead field 404is associated with the resulting voltage measurement electrodes. Asfurther shown in FIG. 4, the lead field associated with the electricalenergy stimulus electrodes 310, 312 and the lead field associated withthe resulting voltage measurement electrodes 420, 308 have substantiallyopposite directions at the left lung 304, and thus illustrate a negativesensitivity electrode arrangement for that target organ.

In brief, the lead fields associated with the electrode configuration ofFIG. 4 negatively intersect in a left lung 304 (see, e.g., phantomencapsulated region 406). Accordingly, when using a constant voltagesource, for example, to inject current, a sensed increase in resultingvoltage indicates an increase in fluid within such region. The constantvoltage source may be implemented as the leading edge of a cardiac orother stimulation pulse derived from discharging a capacitor into thebody. In such an arrangement, impedance is not required to monitor fluidwithin a region of a subject. Rather, more simply, delivering a constanttest voltage and monitoring of a resulting voltage changes on thesensing electrodes may be used. FIG. 5 illustrates a general increase500 in resulting voltage (V_(R)) as time progresses, such as from timeperiod t₁₀₀-t₁₀₈, and thereby (in a negative sensitivity arrangement)indicates an increase in fluid in the thoracic cavity during suchperiod, which may be the result of pulmonary edema 100. Initially, suchas from time period t₀-t₉, FIG. 5 illustrates a substantially stablefluid balance condition as the resulting voltage trends horizontally502.

FIG. 6 illustrates an exemplary block diagram circuit representation ofan IMD 306, such as the IMD of FIGS. 3 and 4. IMD 306 includes circuitsfor, among other things, delivering an electrical energy stimulus,measuring a resulting voltage, and communicating with one or moreexternal devices. An electrical energy output circuit 602 includes apulse generator, which may generate a pulse and deliver such pulsebetween two or more electrodes (e.g., 310, 312 (FIG. 4)), such as aconstant current pulse or a constant voltage pulse, for example. Thiscreates a lead field (e.g., 402 (FIG. 4)) in the body of a subject. Aresponsive voltage may be measured using a voltage sensing circuit 604.In one example, voltage sensing circuit 604 is electrically coupled totwo different electrodes (e.g., 420, 308 (FIG. 4)) than the electrodesused to deliver the electrical energy stimulus. In one such example, theelectrodes used to measure the resulting voltage are positioned such theresponsive lead field opposes the delivered lead field, thereby creatinga negative sensitivity electrode configuration.

A control circuit 606 receives or contains information on the magnitudesof the electrical energy stimulus and the resulting measured voltage. Ananalog-to-digital (referred to as “A/D”) converter may be used totranslate such information into a digitized form used by the controlcircuit 606. A processing unit such as a microprocessor,microcontroller, or digital signal processor within control circuit 606may then use information about the electrical energy stimulus andresulting voltage to calculate a fluid volume indicative signal. In oneexample, the fluid volume indicative signal is calculated solely usinginformation about the resulting measured voltage.

In another example, the fluid volume indicative signal is calculatedusing a product of the resulting measured voltage and the introducedcurrent. In this case, the monitored quantity may be thought of as apartial measure of dissipated power in, for example, the thorax, sincepower dissipated by a resistive load is the product of the currentflowing through it times the voltage it has. In the example of left lung304 located in a negative sensitivity arrangement, the more edema fluidthe lung has, the more the voltage in limbs (for example) increases,such that the power available and measured elsewhere in the body willconsequently be increased as well. This increase in power appearing onthe body can be monitored using, for example, internal can electrode 312and right ventricular electrode 308 (see FIG. 7) to measure the powerappearing in a substantial portion of the subject's thorax. Using thepower to monitor fluid status takes advantage of the synergisticincreases in injected current and resultant voltage that occur whenedema fluid appears in a targeted organ. The multiplication of these twosynergistic quantities amplifies the measurement signal of thedeveloping edema, yielding a more sensitive system to the fluid in thetargeted organ.

As discussed above, when the fluid within a region of a subjectincreases, the resulting measured voltage also increases (assuming anegative sensitivity electrode configuration). Thus, simple monitoringof a resulting measured voltage change in response to a deliveredvoltage pulse may be used to monitor fluid and assess whether pulmonaryedema is present, and if so, a degree of the edema. Thus, the presentIMD 306 need not contain a dedicated current reference circuit to ensurea constant current is delivered between the two or more stimuluselectrodes as is often the case in impedance-based fluid monitoringsystems. Because such current information is not critical for fluidmonitoring purposes according to the present systems and methods (whichrelate to a negative sensitivity electrode configuration), a voltagesource stimulus can be used instead of a current source stimulus, ifdesired.

Control block 606 may additionally include read-only memory (referred toas “ROM”), random-access memory (referred to as “RAM”), flash memory,EEPROM memory, and the like, which may store instructions that may beexecuted by the processing unit, as well as digital-to-analog (referredto as “D/A”) converters, timers, counters, filters, switches, etc.Electrical energy stimulus and resulting measured voltage values mayalso be stored in the memory.

Information from a different source, such as sensor block 608, may beuse to adjust the relationship between the measured resulting voltageand the amount of fluid in the subject. In one example, a posture sensor610 may provide patient orientation or posture information to controlblock 606, allowing posture compensation to be included in theassessment of fluid build-up. Because organs, such as the lungs, andexcess fluid in the thorax and lungs 304 (FIG. 3) shift with posturechanges due to gravity, measured resulting voltage may vary as a subjectassumes different positions. In another example, an activity sensor 612may also provide information to control block 606.

A telemetry block 614 may communicate wirelessly using radio frequency(referred to as “RF”) transmissions over an antenna 616 with a similarlywirelessly equipped external device 618. External device 618 may be acomputer, a home station device, a wearable device, an external weightscale, or any other appropriate device that may be used to program IMD306 or retrieve information from the IMD, such as information about theelectrical energy stimulus or the resulting voltage sensed. One exampleof a suitable external weight scale for use with the present systems andmethods is discussed in commonly assigned Belalcazar, U.S. patentapplication Ser. No. 11/419,120, entitled “MONITORING FLUID IN A SUBJECTUSING A WEIGHT SCALE,” filed May 18, 2006, which is hereby incorporatedby reference in its entirety. Such communication can be used to providethe subject or a caregiver with an alert of a fluid increase indicativeof pulmonary edema or CHF decompensation. Information about thesubject's fluid status may also be used to titrate cardiacresynchronization or other therapy to the subject, where such therapycould be any therapy used to treat fluid accumulation or to regulatecardiovascular function.

FIG. 7 illustrates another exemplary negative sensitivity electrodearrangement. In this example, an implantable apparatus includes an IMD306, a first lead 314, and a second lead 408. First lead 314 extendsfrom a lead proximal end 316, where it is coupled to IMD 306, to a leaddistal end 318 disposed within, over, or about a heart 322 (such as aright ventricle 410) and thereby provides at least one conductive pathfrom IMD 306 to the right ventricle. Second lead 408 extends from a leadproximal end 414 to a lead distal end 416 disposed within, over, orabout heart 322 (such as a left ventricle 412) and thereby provides atleast one conductive path from IMD 306 to the left ventricle. As shown,but as may vary, first lead 314 includes two (implanted) electrodes 308,310 near lead distal end 318, second lead 414 includes one (implanted)electrode 420 disposed near lead distal end 416, and a housing 312 orheader of IMD 306 acts as a fourth (implanted) can electrode by beingconductive or at least partial conductive.

In this way, when IMD 306 provides an electrical energy stimulus (e.g.,a constant voltage source given by, for example, a leading edge part ofan applied pacing pulse via electrical energy output circuit 602 (FIG.6)) and electrodes 420 and 310. This creates a lead field 702. Aresponsive voltage may be measured (i.e., sensed) using electrodes 308and electrode 312, which have an associated lead field 704. In FIG. 4,the lead field associated with the electrical energy stimulus electrodes420, 310 is in an opposite direction to the lead field associated withthe resulting voltage measurement electrodes 308, 312.

FIG. 8 illustrates a flow chart of an exemplary method 800 of monitoringfluid in a region of a subject, such as a thoracic region 302 (FIG. 3).The steps of method 800 may be performed solely by an IMD 306 orcooperatively using the IMD and one or more external devices 618, suchas a computer, a home station device, or a weight scale. In one example,method 800 starts at 802, when a caregiver (remotely) transmits aninstruction via a communication network to monitor fluid status withinthe subject. Upon receiving authorization, home station device 618, forexample, transmits a command to IMD 306 to initiate the fluid monitoringprocedure at 804.

At 806, IMD 306 receives the command and proceeds to inject anelectrical energy stimulus, at 808, across a subject's thoracic region(e.g., by applying a pacing voltage between two electrodes). At 810, theinjected stimulus is optionally measured by IMD 306. At 812, themeasured value of the injected stimulus is optionally telemetered to anexternal device 618 for processing or storing within a memory.

At 814, a resulting voltage is measured by an external device, such as aweight scale device, or by IMD 306. Optionally, at 816, the measuredvalue of the resulting voltage is telemetered from IMD 306 to anexternal device or is telemetered from an external device to IMD 306,depending on where such measurement is taken (i.e., internally orexternally) and where processing of such measurement will take place. At818, the measure of the injected stimulus or the resulting voltage isoptionally received.

At 820, a processing unit integrated with IMD 306 or integrated with anexternal device calculates a fluid indicative signal using informationabout the resulting voltage signal, or by using information about theresulting voltage signal and about the injected stimulus. At 822, thecalculated fluid indicative signal may be stored in a memory. At 824,the calculated fluid indicative signal may be compared to one or moreother calculated fluid indicative signals to detect a change in a fluid.Alternatively or additionally, the calculated fluid indicative signalmay be compared to a predetermined specified threshold value todetermine whether a specified change in fluid amount has occurred, at826. Other statistical techniques, histogram techniques, etc. may alsobe used to recognize a change in fluid status. Based on the operation at826, an alert may be provided at 828 if it is determined that aspecified amount of fluid exists within the subject's thoracic region.At 830, a therapy is adjusted or initiated in response to the operationat 826. Such therapy may be provided in a number of ways, such ascardiac resynchronization therapy, pacing therapy,cardioverter-defibrillator therapy, cardiac rhythm management therapy,dietary therapy, or diuretics. In this example, but as may vary, method800 concludes at or before 832.

CONCLUSION

Pulmonary edema is a serious medical condition in which an excessiveamount of fluid accumulates in a subject's thoracic region, such as thelungs. This condition may (and oftentimes does) result from heartfailure. If it exists, pulmonary edema requires immediate care. While itcan sometimes prove fatal, the outlook for subjects possessing pulmonaryedema can be good upon early detection and prompt treatment.

Advantageously, the present systems and methods may provide enhanced andmore simple monitoring of abnormal fluid build-up in the thoracicregion, and thus may provide more timely and cost effective detection ofthoracic fluid build-up, all in the confines of one's home—withouthaving to make an office appointment or traveling thereto. Suchdetection is made possible by, among other things, internally injectingan electrical energy stimulus through the thoracic region, andcalculating a fluid volume indicative signal. Unlike previous fluidmonitoring systems, calculation of the fluid volume indicative signalaccording to the present systems and methods does not requiredetermining impedances or using a constant current source for theinjected electrical energy. Rather, the fluid volume indicative signalcan be found by delivering a test voltage stimulus that is, for example,constant over a time greater than a few second, and using informationabout the resulting voltage signal, or by using information about theintroduced signal and the resulting voltage signal. The currentinjection stimulus need not be specific to the monitoring sub-system,but can be the common pacing pulse of a cardiac rhythm device. Thisaspect of the invention simplified the circuit of the implanted device.

While the majority of this patent document discusses the monitoring offluid in a thoracic region of subject, the present subject matter is notso limited. The fluid monitoring techniques recited herein may be usedthroughout a subject's body, provided an electrical energy stimulus maybe injected and a resulting voltage signal measured. As used herein, anIMD may include, but is not limited to, cardiac rhythm management(referred to as “CRM”) devices such as pacemakers, cardioverters,defibrillators; cardiac resynchronization therapy (referred to as “CRT”)or coordination devices, drug delivery systems, or any other device orcombination of devices adapted to deliver an electrical stimulationpulse (e.g., a pacing pulse). Additionally, the current injection andvoltage measurement electrode pairs discussed herein may be exchanged(i.e., swapped) yielding equivalent measurements (as supported by theHelmholtz theorem of reciprocity). For instance, an electrode pair usedto inject a current in one example, may be used to measure a resultingvoltage in another example. Likewise, an electrode pair used to measurethe resulting voltage in one example, may be used to inject the currentin another example.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (or aspects thereof) may be used in combination with eachother. Many other embodiments will be apparent to those of skill in theart upon reviewing the above description. The scope of the presentsystems and methods should, therefore, be determined with reference tothe appended claims, along with the full scope of legal equivalents towhich such claims are entitled. In the appended claims, the terms“including” and “in which” are used as the plain-English equivalents ofthe respective terms “comprising” and “wherein.” Also, in the followingclaims, the terms “including” and “comprising” are open-ended, that is,a system, assembly, article, or method that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, various features may be grouped together to streamline thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter may lie in less thanall features of a single disclosed embodiment. Thus the following claimsare hereby incorporated into the Detailed Description, with each claimstanding on its own as a separate embodiment.

1. A method to monitor fluid in a region of interest of a subject, the method comprising: delivering a constant test voltage to create a first lead field in the subject, the first lead field having a first originating location and a first terminating location, the first originating location including a left pectoral region of a thorax of the subject and the first terminating location including a right ventricle of a heart of the subject; measuring a responsive voltage associated with a second lead field in the subject, the second lead field having a second originating location and a second terminating location, the second originating location including a left ventricle of the heart and the second terminating location including the right ventricle of the heart, wherein the first and second lead fields are arranged in a negative sensitivity configuration having substantially opposing field directions at the region of interest, the region of interest located substantially in a left lung region of the subject near the left ventricle and between the first originating location and the second originating location; and calculating a signal indicative of fluid status in the region of interest of the subject using at least the responsive voltage.
 2. The method of claim 1, wherein an orientation of the first lead field differs from an orientation of the second lead field by an angle of more than 90 degrees at the region of interest of the subject.
 3. The method of claim 1, comprising storing a history of the signal indicative of fluid status.
 4. The method of claim 1, comprising detecting a change in fluid volume in the region of interest of the subject using the signal indicative of fluid status.
 5. The method of claim 4, comprising monitoring the signal indicative of fluid status to detect an increase in fluid volume.
 6. The method of claim 5, comprising generating an alert in response to a specified detected increase in the fluid volume.
 7. The method of claim 5, comprising adjusting a therapy to the subject using information from the signal indicative of fluid status.
 8. The method of claim 1, wherein calculating the signal indicative of fluid status in the region of interest of the subject includes using the constant test voltage and the responsive voltage.
 9. The method of claim 1, wherein calculating the signal indicative of fluid status in the region of interest of the subject includes solely using the responsive voltage.
 10. The method of claim 1, comprising determining a fluid status in the region of interest of the subject using at least the responsive voltage.
 11. The method of claim 1, comprising determining a degree of edema in the subject using at least the responsive voltage.
 12. The method of claim 1, wherein measuring the responsive voltage includes measuring a voltage in response to the delivered constant test voltage.
 13. A method to monitor fluid in a region of interest of a subject, the method comprising: delivering a constant test voltage to create a first lead field in the subject, the first lead field having a first originating location and a first terminating location, the first originating location including a left pectoral region of a thorax of the subject and the first terminating location including a right ventricle of a heart of the subject; measuring a responsive voltage associated with a second lead field in the subject, the second lead field having a second originating location and a second terminating location, the second originating location including a left ventricle of the heart and the second terminating location including the right ventricle of the heart, wherein the first and second lead fields are arranged in a negative sensitivity configuration having substantially opposing field directions at the region of interest, the region of interest located substantially in a left lung region of the subject near the left ventricle and between the first originating location and the second originating location; and calculating a signal indicative of fluid status in the region of interest of the subject using the constant test voltage and the responsive voltage.
 14. The method of claim 13, comprising determining a degree of edema in the subject using the measured responsive voltage.
 15. The method of claim 13, comprising generating an alert in response to a specified detected increase in the fluid volume.
 16. The method of claim 13, comprising adjusting a therapy to the subject using information from the signal indicative of fluid status.
 17. A method to monitor fluid in a region of interest of a subject, the method comprising: delivering a constant test voltage to create a first lead field in the subject, the first lead field having a first originating location and a first terminating location, the first originating location including a left pectoral region of a thorax of the subject and the first terminating location including a right ventricle of a heart of the subject; measuring a responsive voltage associated with a second lead field in the subject, the second lead field having a second originating location and a second terminating location, the second originating location including a left ventricle of the heart and the second terminating location including the right ventricle of the heart, wherein the first and second lead fields are arranged in a negative sensitivity configuration having substantially opposing field directions at the region of interest, the region of interest located substantially in a left lung region of the subject near the left ventricle and between the first originating location and the second originating location; and calculating a signal indicative of fluid status in the region of interest of the subject solely using the responsive voltage.
 18. The method of claim 17, comprising determining a degree of edema in the subject using the measured responsive voltage.
 19. The method of claim 17, comprising generating an alert in response to a specified detected increase in fluid volume.
 20. The method of claim 17, comprising adjusting a therapy to the subject using information from the signal indicative of fluid status. 